The application of numerical methods to the study of the distribution of trace fossil in hardground surfaces appears scarcely in the previous bibliography, mainly because these traces usually have a random distribution not controlled by notable geometrical patterns. The good appearance of trace fossils in the Mo/a unit outcrops, and their coexistence with joints, have determinated their study with quantitative geostatistical methods, which have shown the absence of a tectonic or sedimentological control in their genesis. It is not posible to establish any correlation between the length and the direction of the trace fossils, that indicates control by neither previous fractures nor marine currents, but homogeneous conditions of oxigenation and bathymetry in the substrate and bottom marine waters